1. Field of Invention
The present invention relates to power conversion, and more particularly, to a system and method for alternating current (AC) voltage regulation.
2. Description of Related Art
Power converters are essential for many modern electronic devices. Among other capabilities, power converters can adjust power level downward (buck converter) or adjust power level upward (boost converter). Power converters may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. Further, power converters can regulate power, for example, voltage regulation of DC to DC or AC to AC. An AC-to-AC voltage regulator can produce a controlled AC output voltage from a varying AC input voltage.
Previously developed AC-to-AC voltage regulators may employ a bi-directional switch for providing the controlled AC output. Such bi-directional switch may comprise one or more devices such as silicon controlled rectifiers (SCRs) or thryristors, or transistors (e.g., IGBTs or MOSFETs) and diodes. If the bi-directional switch is implemented with SCRs, then the AC-to-AC voltage regulator can only use a single pulse leading edge control scheme. This type of control will generate very high total-harmonics-distortion (THD).
THD can be reduced by controlling the bi-directional switch with pulse width modulation (PWM) to “chop” the AC input waveform during, for example, one half cycle for an AC-to-AC voltage regulator. For this case, thryristors or SCRs cannot be used for the bi-directional switch. Instead, the bi-directional switch must be implemented using transistors. Ideally, if the load is resistive only and line reactance is zero, then multiple chopping can be performed on the AC input waveform without problem. However, no ideal load exists in the real world and there will always be line inductance. This generates very high voltage spikes across the transistors in the bi-directional switch. The voltage spikes produce a lot of electromagnetic interference (EMI) and distortion and, even worse, can cause the transistors in the bi-directional switch to avalanche, ultimately destroying them.